You use the CUTPOINT option of the MONTECARLO command. See Example 11.1 in the Mplus User's Guide. The value given is a z-score. So you use a z-score table to select the value that corresponds to a 75/25 split.

I have a similar question about generating a binary independent variable in a Monte Carlo simulation. I'd like to generate "Gender" variable that with population mean of 0.5 and variance of 0.25. What code should I use? Thank you in advance.

I am doing a Monte Carlo simulation of a growth model with 3 time points and a nominal predictor (3 intervention groups). I specified 2 binary dummy variables (x2 and x3) to represent my 3 groups. What CUTPOINT value should I use for these 2 dummy variables, considering they are not a 50/50 split (1/3 of sample gets intervention A, 1/3 gets intervention B, and 1/3 gets intervention C)?

Thank you, that was very helpful. In general, we expect to see that 2 of the 3 groups do not change over the 3 time points, while the third group improves on the outcome. Would you say that the use of 2 dummy variables is an accurate way to estimate the necessary sample size (does the simulation know there are 3 linked groups with these 2 dummy variables or is it only doing a 2-group comparison)?

On a related note, would you suggest setting the slope growth factors at different values to reflect our hypothesis, and reference the power associated with the smaller parameter estimate to determine sample size?

Actually, you are better off doing a multiple-group analysis with 3 groups, where you control the number of observations in the groups and have freedom to vary any parameter across the groups. So don't use dummy variables.

Your hypothesis sounds like you would have the slope mean at zero in two of the groups.